<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>倒计时效果</title>
    <style>

        span {
            /* 变成行内块元素 */
            display: inline-block;
            width: 40px;
            margin-left: 5px;
            background-color: black;
            color: #fff;
            text-align: center;
            line-height: 40px;
        }
    </style>
</head>
<body>
    <div>
        <span class="hour"></span>
        <span class="minute"></span>
        <span class="secoud"></span>
    </div>
    <script>
        var hour = document.querySelector('.hour')
        var minute = document.querySelector('.minute')
        var secoud = document.querySelector('.secoud')
        var inputTime = +new Date('2021-03-22 19:00:00')// 返回用户输入时间的总毫秒数
        countDown() //先调用一次函数 房子页面刷新
        // 开启定时器
        setInterval(countDown,1000)
        function countDown() {
            var nowTime = +new Date() //返回的是当前时间的总毫秒数
            var times = (inputTime - nowTime) / 1000 //剩余的总秒数
            var h = parseInt(times / 60 / 60 % 24)// 时
            h = h < 10 ? '0' + h : h;
            hour.innerHTML = h
            var m = parseInt(times / 60 % 60)// 分
            m = m < 10 ? '0' + m : m;
            minute.innerHTML = m
            var s = parseInt(times % 60)//秒
            s = s < 10 ? '0' + s : s;
            secoud.innerHTML = s

        }
    </script>
</body>
</html>